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Re: [PATCH][AArch64] PR71951: Fix unwinding with -fomit-frame-pointer


From: Wilco Dijkstra
Sent: 31 July 2017 16:57
To: GCC Patches; James Greenhalgh
Cc: nd
Subject: [PATCH][AArch64] PR71951: Fix unwinding with -fomit-frame-pointer
As described in PR71951, if libgcc is built with -fomit-frame-pointer,
unwinding crashes, for example while doing a backtrace.  The underlying
reason is the Dwarf unwinder does not setup the frame pointer register
in the initialization code.  When later unwinding a function that uses
the frame pointer, it tries to read FP using _Unwind_GetGR, and this
crashes if has never restored FP.  To unwind correctly the first frame
must save and restore FP (it is unwound in a special way so that it
uses SP instead of FP).  This is done by adding -fno-omit-frame-pointer.

OK for commit and backport to GCC6/7?

2017-07-31  Wilco Dijkstra  <>

        PR target/71951
        * config/aarch64/aarch64.h (LIBGCC2_UNWIND_ATTRIBUTE): Define.

diff --git a/gcc/config/aarch64/aarch64.h b/gcc/config/aarch64/aarch64.h
index 7f91edb5713d7e8eda2f0a024a0f97b4e111c4b0..03fd93046bdbdb03bd7d0c4573928f504640f7e1 100644
--- a/gcc/config/aarch64/aarch64.h
+++ b/gcc/config/aarch64/aarch64.h
@@ -971,4 +971,12 @@ extern const char *host_detect_local_cpu (int argc, const char **argv);
 extern tree aarch64_fp16_type_node;
 extern tree aarch64_fp16_ptr_type_node;
+/* The generic unwind code in libgcc does not initialize the frame pointer.
+   So in order to unwind a function using a frame pointer, the very first
+   function that is unwound must save the frame pointer.  That way the frame
+   pointer is restored and its value is now valid - otherwise _Unwind_GetGR
+   crashes.  Libgcc can now be safely built with -fomit-frame-pointer.  */
+  __attribute__((optimize ("no-omit-frame-pointer")))
 #endif /* GCC_AARCH64_H */

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